The present invention relates to die casting equipment and more particularly to a shot sleeve through which the molten metal is transferred into the die.
Die casting is a common used technology for manufacturing metal articles. Typically, the die casting apparatus includes a pair of die halves each formed with a void corresponding to a portion of the article to be cast. When the two die halves are brought together in proper alignment, their respective voids cooperate to form a die cavity corresponding to the shape of the article to be cast. Molten material is introduced into the die and allowed to cure--typically by cooling the molten material to allow it to solidify. Once the material is sufficiently cured, the die halves are opened and the cast article is removed.
The die cast machine includes a shot sleeve to inject the molten metal into the die cavity. The shot sleeve defines an internal bore communicating with the die cavity. A plunger reciprocates within the shot sleeve to inject or force the molten metal into the die cavity. The plunger is connected to a hydraulic cylinder by a plunger rod. Extension of the plunger injects the molten metal within the sleeve into the die cavity. Retraction of the plunger withdraws the plunger to permit filling the sleeve for the next shot.
It is desirable to match the volume of the shot sleeve to the amount of metal required for a single shot into the die. Accordingly, the outer end of the shot cylinder includes a threaded stroke adjuster, which permits fine tuning of the position of the retracted plunger and therefore the volume of the shot cylinder. Turning the stroke adjuster makes small changes in the stroke length and consequently the internal volume of the shot sleeve. These minor adjustments may be necessary, for example, to compensate for expansion or contraction of the die: components or for slight variations from design specifications.
During the first shots as a die casting machine is first used, the die casting mold warms up to a proper operating temperature. Because an inadequately heated die produces low quality castings, the articles cast during warm-up are either recycled or scrapped. Obviously, this process is undesirably wasteful and costly, but cannot be avoided.
Die cast techniques vary in part depending on the desired strength of the article. One technique, providing comparable strength and wear with lighter weight than cast iron and steel articles, is to include a structural insert in an aluminum die cast article. Such inserts are typically formed from cast iron, steel, or precast aluminum and are placed in the die cavity prior to die casting so that they are encapsulated by moltan material to become an integral part of the article. For example, an insert may be located in a high stress portion of an article to bolster the casting or along contact surfaces to prevent coining or wear of the article.
When inserts are used, they are wasted during die warm-up. As noted above, articles cast during the warm-up period are discarded or recycled because they are of inferior quality. Consequently, the inserts used during the warm-up castings also are discarded. This is both wasteful and expensive, because the inserts usually comprise a significant portion of the cost of the cast article.
Conventional die casting equipment undergoes a pressure spike at the end of each shot when the die cavity is filled as the shot plunger continues to move forward. This pressure spike is distributed throughout various die casting components and potentially leads to metal fatigue and ultimately failure. One known method for cushioning this pressure spike is to provide a relief hydraulic cylinder having a relief plunger that extends into the gate at the junction of the shot sleeve and the die. When the die cavity is filled, the metal pushes the relief plunger backwards in the cylinder. Immediately following this cushioning, the relief cylinder returns the relief plunger to its original position. This method requires additional components and machining, decreases the reliability of the die caster, and adds additional conventional problems associated with the fluid metal in contact with the plunger.